Seasonal variation of the oxygen isotopic ratio of atmospheric carbon dioxide in a temperate forest, Japan

Abstract

[1] In order to examine the role of terrestrial biospheric activity in the seasonal variation of d 18 O of atmospheric CO 2 , we measured d 18 O values of CO 2 and H 2 O in the soil and the canopy air in a temperate forest in the northern part of the main island of Japan for the period August 2000-December 2001. It was found that the d 18 O value of the soil water near the surface varied seasonally almost in phase with that of precipitation, except for spring. On the other hand, its seasonal variation at 70 cm depth showed a 3-month delay compared to those at 10 and 40 cm depths, owing to slow penetration of precipitation into the soil. The measured d 18 O values of CO 2 in the soil air at 10, 40, and 70 cm depths agreed well with the results calculated under the assumption that CO 2 in the soil air was isotopically equilibrated with its surrounding soil water. On the basis of the measurement results, we derived the seasonally varying oxygen isotopic discrimination factors for respiration and photosynthesis processes. Using these discrimination factors as well as biospheric CO 2 fluxes estimated by employing an ecosystem model, we calculated the seasonal variation of d 18 O of atmospheric CO 2 in the forest canopy. The result showed a good agreement with the observation, indicating that the isotopic contribution of photosynthesis to the atmospheric d 18 O was strong in spring and fall, while that of respiration was dominant in summer and winter.

Application of a laser-based spectrometer for continuous in situ measurements of stable isotopes of soil CO<sub>2</sub> in calcareous and acidic soils

“…δ 18 O depth patterns of soil CO 2 in the acidic soil most likely reflected δ 18 O values of soil water as CO 2 became increasingly 18 O depleted from top to bottom. The δ 18 O of deeper soil layers CO 2 (30-60 cm) was close to the values expected when full oxygen exchange between soil water and CO 2 occurred (Kato et al, 2004). Assuming an 18 O fractionation of 41 ‰ between CO 2 and water (Brenninkmeijer et al, 1983), this would result in an expected value for CO 2 of ≈ −10 ± 2 ‰ vs. VPDB CO 2 .…”

“…The conventional method for estimating the δ 13 C and δ 18 O of soil CO 2 efflux is by using two end-member mixing models of atmospheric CO 2 and CO 2 produced in the soil (Keeling, 1958). The conventional methods for sampling soil produced CO 2 are chamber-based (Bertolini et al, 2006;Torn et al, 2003), "mini-tower" (Kayler et al, 2010;Mortazavi et al, 2004), and soil-gas-well-based (Breecker and Sharp, 2008;Oerter and Amundson, 2016) methods. In conventional methods, air sampling is done at specific time intervals, and δ 13 C and δ 18 O are analysed using isotope ratio mass spectrometry (IRMS; Ohlsson et al, 2005).…”

Seasonal variations of atmospheric CO2, δ13C, and δ18O at a cool temperate deciduous forest in Japan: Influence of Asian monsoon

“…[35] There are many processes that influence the observed d 18 O-CO 2 variation at TKY on a seasonal time scale [e.g., Farquhar et al, 1993;Tans, 1998;Cuntz et al, 2003;Kato et al, 2004]. One such process that takes place in a biologically active area, such as the TKY site, is the retrodiffusion of CO 2 isotopically equilibrated with leaf water to the atmosphere during photosynthesis and the emission of respired CO 2 diffused out of soil after isotopic exchange with soil moisture [Peylin et al, 1999].…”

“…However, many of these isotopic studies have focused on diurnal and/or short-term phenomena based on intensive observation campaigns over a relatively short period [e.g., Yakir and Wang, 1996;Flanagan et al, 1996Flanagan et al, , 1997Bowling et al, 2001Bowling et al, , 2002Langendörfer et al, 2002]. Combined long-term isotopic and CO 2 concentration measurements, with an aim at identifying processes controlling seasonal and interannual variations, are very few [Bakwin et al, 1998b;Levin et al, 2002;Welp et al, 2006], especially in the monsoon region in Asia [Nakazawa et al, 1997b;Kato et al, 2004;Uchida et al, 2008].…”

Observation of O2:CO2 exchange ratio for net turbulent fluxes and its application to forest carbon cycles

“…However, large uncertainties are still associated with estimating RE using an empirical function of air or soil temperature, thus causing similarly large uncertainties in GPP. Measurements of d 18 O of atmospheric CO 2 have been used as an alternative method to elucidate the forest carbon cycle (e.g., Kato et al 2004); however, complex discrimination processes of d 18 O during respiration and photosynthesis make the precise separation of GPP and RE difficult using this approach. Considering these facts, we investigated the possibility of using the O 2 /N 2 ratio to elucidate the forest CO 2 cycle.…”